The assignee of the present invention manufactures and deploys spacecraft for, inter alia, communications and broadcast services. Such spacecraft carry on board propulsion systems, including a set of thrusters, that may be configured to perform velocity change maneuvers (“delta-V” maneuvers) as well as provide control torques under control of a spacecraft attitude control system. For example, geosynchronous spacecraft are typically required to remain within an assigned orbital station or “box” which has predetermined dimensions. The act of maintaining a geosynchronous spacecraft in its assigned station is referred to as station-keeping. For example, a type of delta-V maneuver referred to as a north-south station-keeping (NSSK) maneuver may be periodically required to control the inclination of the spacecraft's orbital plane with respect to the Earth's equatorial plane. As a further example, spacecraft in low earth orbit may require periodic delta-V maneuvers to compensate for atmospheric drag.
Specific attitudes of body-stabilized satellites with respect to the earth must be maintained in the face of external disturbance torques, including, for example, solar pressure, gravity gradients and magnetic fields. These disturbance torques are typically countered with correction torques generated by exchanging momentum between a satellite body frame and one or more reaction wheels or momentum wheels by changing the rotation rate of the reaction/momentum wheels. Because such wheels have a rotation rate design limit, momentum management techniques are required whereby their stored momentum is periodically desaturated (“unloaded” or “dumped”) by applying external control torques to the satellite. These control torques may be provided by firing one or more thrusters that are directed such that a resulting thrust vector defines a moment arm with respect to the satellite's center of mass.
Station-keeping and momentum management techniques are described, for example, in U.S. Pat. Nos. 4,767,084, 6,296,207, and 6,032,904, assigned to the assignee of the present invention, the disclosures of which are incorporated by reference into the present application for all purposes.
Improved techniques that reduce the propellant required for station-keeping are desirable, so as to increase spacecraft life and/or reduce launch costs.